1. Field of the Invention
The present invention relates to a wide area laser and a laser optical cavity design for wide area lasers, and more particularly to a laser and a laser cavity having multiple off-axis oscillation paths.
2. Description of Related Art
Over the past several years, laser technology has become commonplace in a large number of military, industrial, research, medical and consumer applications. The widespread usage for lasers has increased the need for development of compact lasers that are reliable, durable and relatively inexpensive. Looking at one of the more widely used lasers, the CO2 laser, conventional lasers capable of delivering substantial power have the disadvantages that high voltage D.C. is required for excitation, series ballast resistors are usually required to stabilize the discharge, the discharge tube may be long and fragile, and length scaling is required, with increased power being achieved by lengthening the optical cavity, all of which limit the availability and ease of use of the lasers. Other types of gas lasers are subject to similar disadvantages and limitations.
Area-scaled lasers first received attention in the mid- to late 1980s as being more easily excitable by radio-frequency (RF) discharges, diffusion-cooled, and scalable to achieve the desired power by varying the excitation electrode area. Slab, or wide area, laser cavities, comprising parallel planar electrodes, provide the advantage that the width of the electrode can be increased to increase the extractable power, thus minimizing the need for the length scaling that is required in prior art gas lasers. (See, e.g., K. M. Abramski, et al., xe2x80x9cPower Scaling of Large-Area Transverse Radio Frequency Discharge CO2 Lasersxe2x80x9d, Appl. Phys. Lett. 54 (19), May 8, 1989.) Assuming that the significant amount of heat generated by the large volumetric excitation can be dissipated, the issue of extracting the laser radiation in a useful form still remains. For most purposes, the optimal cross-section of a laser beam has a gaussian distribution, i.e., in the lowest order, TEM00, transverse mode. Alternatively, a flattened gaussian distribution, xe2x80x9csuper gaussianxe2x80x9d, a donut mode, or a superposition of several lowest order modes, are all useful for industrial and medical purposes. For military sensors, lowest order gaussian or xe2x80x9csuper gaussianxe2x80x9d modes are essential or highly desirable. Regardless of the application, conventional laser optical cavities cannot be used to extract energy from wide area discharges since such techniques yield highly multimode outputs which cannot be well collimated or tightly focused. While military sensors cannot use multimode lasers, such lasers are also undesirable or less desirable for industrial cutting, drilling, scribing, welding, etc. Even medical applications, which are more tolerant of multimode laser radiation, are better served by low order gaussian or donut modes which simplify the optical system that transports the laser to the tissue.
Considerable attention has been directed toward extracting the laser radiation from large area cross-sections in useful intensity profiles. Many workers in this area have used unstable resonators, including, for example, Opower (U.S. Pat. No. 4,939,738), Hobart, et al. (U.S. Pat. No. 5,123,028), Koop, et al. (U.S. Pat. No. 5,353,297), P. E. Jackson, et al., xe2x80x9cCO2 Large-Area Discharge Laser Using an Unstable-Waveguide Hybrid Resonatorxe2x80x9d, Appl. Phys. Lett. 54 (20), May 15, 1989. See, also, A. E. Siegman, Lasers, University Science Books, 1986, Chapters 21-23. The resonators disclosed by Opower and Jackson, et al utilize a concave mirror at one end of the slab discharge and a convex mirror at the other end, just enough off-center to permit output coupling near the edge of the discharge slab. The electrodes are spaced apart by a small enough spacing, e.g., less than 3 mm, to create a waveguide in the electrode direction, leaving free space propagation in the wide direction. However, such an approach may be feasible only for larger (higher power) lasers.
Historical Development
With the exception of unstable resonators, most commercial lasers utilize optical cavities that were first analyzed and employed in the early 1960""s. Such laser cavities, in which a beam retraces itself along a single axis after being reflected by a flat mirror at each end of the cavity, is a flat plate Fabry-Perot multibeam interferometer. Spherical mirrors are more commonly used in a version of the Fabry-Perot interferometer, confining the intracavity laser mode as shown in FIG. 8, where the values of w0 and w1, are the beam radius at the center and at the mirrors, respectively.
Other early forms of interferometers were configured to cause the beam to make one round trip along several different axes before returning to its starting point. For example, in the confocal Fabry-Perot interferometer illustrated in FIG. 9a, the beam makes two round trips, encountering four different mirror points. In a focal spaced Fabry-Perot interferometer, such as shown in FIG. 9b, a ray makes three round trips, encountering six points on the mirror surfaces. There is a large number of multi-beam interferometers in which a plurality of off-axis cavity round trips and mirror encounters occur before the beam returns, in phase, to its starting point. Such off-axis spherical mirror interferometers have been investigated for use in laser amplifiers (see, e.g., Herriot, et al., xe2x80x9cOff-axis paths in spherical interferometersxe2x80x9d, Appl. Optics, 3, p. 523, April 1964, and Herriot, et al., xe2x80x9cFolded optical delay linesxe2x80x9d, Appl. Optics, 4, p. 883, August 1965) and as Raman gain cells (Trutna, et al, xe2x80x9cMultiple pass Raman gain cellxe2x80x9d, Appl. Optics, 19, p. 301, Jan. 15, 1980). In such optical resonators, for example, when a beam makes three round trips in a cavity before returning to its starting point, the cavity path is three times as long as that of the flat plate Fabry-Perot interferometer, and the cavity xe2x80x9cfree spectral rangexe2x80x9d, i.e., the frequency spacing between adjacent longitudunal modes, is one-third as large. Such off-axis long path cells are often referred to as xe2x80x9cWhitexe2x80x9d cells (based on 1941 research in connection with increasing the sensitivity of absorption measurements), but are also known as xe2x80x9cHerriotxe2x80x9d cells.
The off-axis interferometer configuration has also been used for a laser cavity, as disclosed by J. G. Xin and D. R. Hall in xe2x80x9cCompact, Multipass, Single Transverse Mode CO2 Laserxe2x80x9d, Appl. Phys. Lett. 51 (7), Aug. 17, 1987. In this laser, the beam xe2x80x9cwalkedxe2x80x9d around an annular region between coaxial plates, achieving excellent transverse mode (near TEM00) discharge and about 65 watts output. However, there are several drawbacks to the coaxial waveguide, including that it is difficult to both support and align the coaxial electrode and to feed it with RF energy. It is also difficult to align the cavity mirrors to allow the beam to optically xe2x80x9cwalkxe2x80x9d around the annular region.
Accordingly, the need remains for a laser optical cavity design which is appropriate for use over a wide range of power levels which is durable and relatively easy to construct, and which is capable of providing power level and efficiencies equivalent to conventional laser many times longer.
It is an advantage of the present invention is to provide a highly compact gas laser which uses wide area electrodes in an off-axis, multi-pass resonator to create a long effective gain path with a short physical length.
It is another advantage of the present invention to provide a laser cavity design which effects a hybrid mode pattern, permitting extraction of useful laser radiation from wide area excitation.
Another advantage of the present invention is to provide a laser cavity design which allows use of a short physical length with a multi-path beam to produce laser radiation from wide area excitation.
Yet another advantage of the present invention to provide a stable optical resonator to extract high power, high efficiency laser radiation from a short, compact cavity.
Still another advantage of the present invention is to provide a carbon dioxide laser which produces a hybrid mode laser beam which is a combination of waveguide mode perpendicular to the electrode plates and free space mode parallel to the plates.
In an exemplary embodiment, a laser includes a pair of electrode plates which are RF excited transversely and an optical resonator defined by a pair of mirrors. For use in a sealed CO2 laser, the electrodes are separated by an electrode spacing such that the modes perpendicular to the electrodes are waveguide in nature. The close spacing of the electrodes provides not only beam guidance, but also efficient thermal diffusion cooling. Due to the relationship between gas pressure and spacing (Pressurexc3x97spacing=constant), a close spacing allows for higher optimum pressures, giving a higher saturation intensity for higher power as well as a broader gain line. This, combined with the closely spaced longitudinal modes assures more stable amplitude operation. Modes parallel to the electrodes are free space in nature, providing hybrid propagation. In other types of lasers, a similarly-configured cavity operates in free space mode in both directions. Spherical mirrors are disposed near the ends of the electrodes to form the optical resonator. The mirrors are substantially identical except that one of the mirrors has a small transmissive aperture for coupling laser energy out of the cavity. The precise radius of curvature of each mirror and the precise distance between the two mirrors are selected to cause a laser beam to traverse the resonator with a plurality of off-axis traversals to trace out a plane through the gain region before exiting through the aperture in the first spherical mirror. The beam radii within the resonator and at the mirrors for each of the free space and the waveguide modes should be as close as possible to each other. Similar principles may be applied to a laser crystal to provide off-axis, multi-beam operation in appropriate solid state lasers.
In an exemplary embodiment of a CO2 laser, a vacuum-tight housing and an electrode pair are made of high purity copper. The electrode spacing is small enough to act as a waveguide, e.g., less than 3 mm. An RF feed-through with a copper conductor passes through the housing to provide electrical connection with one of the electrodes. The sidewalls of the discharge region are made from beryllia. Stainless steel end plates and mirror mounts support the spherical mirrors a short distance from the ends of the electrodes. Copper cooling chambers are attached to the housing exterior. In addition to guiding the beam, the close spacing of the electrodes facilitates efficient gas cooling by way of thermal diffusion to the electrode plates. Higher power is achieved primarily by increasing the electrode width to widen the RF discharge, although some lengthening may be employed as well. The mirror separation and curvature are selected to provide a plurality of cavity traversals such that, after the plurality of traversals, wherein a linear plane is traced out through the gain region, the beam returns to its starting point for coupling out of the cavity through a transmissive aperture in one of the mirrors.